1. Field of the Invention
The present invention relates to a printing apparatus and a driving method therefor.
2. Description of the Related Art
A printing apparatus includes, for example, a printhead for printing on a printing medium, and a conveying roller for conveying the printing medium. A plurality of nozzles for printing dots by discharging ink are provided in an inkjet printhead, and each nozzle performs preliminary discharge at a predetermined interval. Preliminary discharge is one of recovery processes of the printhead to prevent clogging of each nozzle. Since nozzles whose driving frequency is relatively low can appropriately print dots by performing preliminary discharge, the quality of an image formed on a printing medium is improved.
Japanese Patent Laid-Open No. 2005-246643 discloses a technique of performing “preliminary discharge on a sheet” to print dots by preliminary discharge on a printing medium at a density which has no influence on the visibility. Preliminary discharge on a sheet is advantageous in, for example, improving the image quality in an arrangement in which printing is executed on a printing medium such as a longitudinally long-shaped sheet (roll sheet) using a full-line printhead while conveying the printing medium.
In some printing apparatuses, a printhead includes two or more nozzle arrays which are used to print dots of the same color and each of which has a plurality of nozzles arranged along a predetermined direction. Print data are distributed to the respective nozzle arrays, and each nozzle array is driven based on the distributed print data. This arrangement is advantageous in improving the print speed since the two or more nozzle arrays are driven in parallel to print dots according to the print data.
Japanese Patent Laid-Open No. 2012-30594 (e.g. FIG. 8C) discloses a technique in which the nozzles of each group of two nozzle arrays are time-divisionally driven, and each nozzle array is time-divisionally driven by shifting the driving timings by a ½ period of time-divisional driving. Similarly, Japanese Patent Laid-Open No. 2012-30594 (e.g. FIG. 11C) discloses a technique in which the nozzles of each group of four nozzle arrays are time-divisionally driven, and each nozzle array is time-divisionally driven by shifting the driving timings by a ¼ period of time-divisional driving.
In the arrangement described in Japanese Patent Laid-Open No. 2012-30594, however, when the conveying speed of a printing medium is increased to improve the throughput, it is necessary to increase the operation speed of each nozzle array (for example, the driving frequency of each nozzle array) so that dots are appropriately printed on the printing medium at a speed corresponding to the conveying speed. This requires the user to change a design of hardware such as the circuit design of a printing apparatus along with a change in operation speed, leading to an increase in manufacturing cost.
Furthermore, in the arrangement described in Japanese Patent Laid-Open No. 2012-30594, when time-divisionally driving each nozzle array, one period of time-divisional driving corresponds to a plurality of column data, and thus a nozzle array capable of printing dots on the printing medium is limited (a nozzle array to be used is fixed for each pixel). At this time, it may be impossible to perform preliminary discharge on sheet by a desired nozzle. Therefore, if data for preliminary discharge in which only usable nozzles are selected are generated in advance and stored in the memory of the printing apparatus in order to make it possible to perform preliminary discharge on a sheet by all nozzles, a large-capacity memory is required to store data for preliminary discharge of the entire print region for each ink color or each nozzle array, leading to an increase in manufacturing cost.